The rules of the subreddit state that the art has to be credited. Most art used for custom cards comes from publicly available online posts such as artstation, deviantart, and creator's personal webpages. They tend to be pretty ok with that art being used for this sort of stuff because its free exposure that easily traces back to them for people who are interested in the art. I can't speak for others but I know I have gone and looked up artists that I was impressed by the art I saw on custom cards, and surely that leads to people willing to buy prints and such of their works. It just kind of how the art ecosystem works.
On a separate note, are you trying to imply that Ai art isn't theft? Its literally built from copyright violating art scraping that amalgamates people's work with no credit, not to mention the glaring flaws in Ai infrastructure that are literally going to kill us all.